Waveguide expansion joint

ABSTRACT

An expansion joint for a waveguide transmission system utilizes a dielectric member to transfer the signal across the joint in the form of a trapped mode and a flexible mechanical closure such as a bellows to permit relative movement between the coupled waveguide sections.

I United States Patent 1151 3,660,788 Alsberg 51 May 2, 1972 [54] WAVEGUIDE EXPANSION JOINT 2,564,007 8/1951 Hochgraf.. .....333/98 2,571,021 10/1951 Early 333/95 A [72] Invent N i Alsberg Berkeley 2,762,981 9/1956 Morgan, 11.. ..335/21 R 2,802,994 8/1957 Ober et a1... ..333/98 R [73] Assignee: Bell Telephone Laboratories, Incorporated, 2,883,632 4/1959 St et ..333/98 Murray Hill, 2,899,651 8/1959 Lanciani ..333/9s BE [22] Filed: Sept. 4,1970 3,087,129 4/1963 Maury et al ..333/98 R PP N95 69,579 Primary Examiner-Herman Karl Saalbach Assistant Examiner-Wm. H. Punter 52 us. 01 ..333/98 R, 333/21, 333 95 A, Guemher and Edwin Cave 333/97 R [51] Int. Cl ..I-IOIp 1/06, HOlp 1/16, HOlp 1/30 ABSTRACT [58] Field of Search ..333/98 R, 98 BE, 95 A, 21 R An expansion joint for a waveguide transmission System mm 56 R f d izes a dielectric member to transfer the signal across the joint 1 e erences I e in the form of a trapped mode and a flexible mechanical clo- UNITED STATES PATENTS sure such as a bellows to permit relative movement between the coupled waveguide sections. 2,892,987 6/1959 Cedrone ..333/98 R 3,001,160 9/1961 Trousdale ..333/98 P 6 Claims, 4 Drawing Figures 'il'llQ'l!!! 222m ,1, 2, 1, 4 2 4M l I v i 1 lmmw\\\\'\\\\\ PATENT'EDMAY 21972 sum 1 er 2 WAVEGUIDE EXPANSION JOINT BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to waveguide transmission systems and more particularly to expansion joints for joining sections of waveguide.

2. Description of the Prior Art Because of temperature changes in the surrounding environment, means must be provided to accommodate expansions and contractions of the rigid waveguide sections used in a waveguide transmission system. The expansions and contractions can be reduced by such methods as prestressing the waveguide sections and by providing buckling restraints. However, even with the use of such methods, expansion joints are still required.

One presently used expansion joint utilizes mating tapered sleeves having a slip fit to join the waveguide sections. At best this type of sleeve presents discontinuties or step junctions to the transmitted signals and thereby degrades the signal through the generation of spurious or unwanted modes.

Accordingly, it is an object of this invention to provide an improved flexible section or expansion joint for waveguide.

Another object is to provide an expansion joint which permits substantial relative motion of the waveguide sections without substantially degrading the signal transmitted therethrough.

A still further object is to provide an expansion joint which will permit substantial axial motion between waveguide sections while maintaining the axial alignment of such sections and transmitting the signal without substantial degradation.

SUMMARY OF THE INVENTION The foregoing objects and others are achieved in accordance with the principles of this invention by the use of a dielectric member to transfer the signal across the joint in the form of a trapped mode and a flexible mechanical closure to permit relative axial motion of the coupled waveguide sections. Specifically, a properly shaped dielectric member bridges the expansion gap and is inserted into the ends of the coupled waveguide sections and concentrically suspended therein. The ends of the waveguide sections are then joined by a flexible member such as a bellows which permits relative axial motion of the joint sections. Energy being transmitted in the sending waveguide, which is usually in the TE mode, is converted into a trapped mode which propagates almost entirely within the dielectric member across the expansion gap. The energy is then reconverted from the trapped mode to the desired transmitting mode for further transmission along the receiving waveguide section. By proper design of the configuration of the dielectric member and proper location thereof in the ends of the waveguide sections, degradation of the transmitted signal can be made very low. Depending upon the desired trapped mode the dielectric member may be a dielectric rod, a dielectric sleeve, or an artificial dielectric sleeve created by the use of dipoles on thin dielectric membranes.

DESCRIPTION OF THE DRAWING The invention will be more fully comprehended from the following detailed description and accompanying drawing in which:

FIG. 1 is a schematic sectional representation of an expansion joint utilizing a dielectric sleeve.

FIG. 2 is a schematic sectional representation of an expansion joint utilizing an artificial dielectric sleeve.

FIG. 3 is a schematic representation of a membrane utilized DETAILED DESCRIPTION Referring now to FIG. 1, waveguide sections 2 and 4 are joined by an expansion joint generally referred to as Q according to one embodiment of this invention. Waveguide sections 2 and 4 will be subjected to changes in the surrounding environment and thus undergo expansions and contractions in the axial direction. These axial movements can be accommodated without degrading the signal transmitted through the waveguide sections by using the expansion joint of this invention.

The ends of waveguide sections 2 and 4 are terminated short of each other thereby leaving a gap 8 which is sufficient to accommodate the axial movements. A flexible mechanical closure such as a bellows l bridges the expansion gap 8 and forms a tight vapor-proof joint with the ends of waveguide sections 2 and 4. Bellows l0 permits axial movements of waveguide sections 2 and 4 while simultaneously isolating the waveguide interior from the surrounding environment. Bellows 10 advantageously can be made from a metal which can be soldered to the ends of the waveguide sections 2 and 4.

A rigid sleeve 12 of suitable dielectric material also bridges 7 expansion gap 8 and the ends thereof are inserted a substantial in the artificial dielectric sleeve of FIG. 2 near the end thereof;

distance into the ends of adjacent waveguide sections 2 and 4. Sleeve 12 is concentrically suspended or mounted in sections 2 and 4. Sleeve 12 can be mounted by annular members 14 which encircle sleeve 12 and are attached to the outer periphery thereof and to the inner periphery of waveguide section 4. Because of the small weight of sleeve 12, the suspension thereof from one waveguide section will normally be sufficient to maintain the concentricity of the sleeve with respect to the axes of both sections. Such suspension permits axial motion of sleeve 12 with respect to the waveguide sections 2 and 4. Annular members 14 may comprise membranes, washers, and the like made of a suitable dielectric material. Annular members 14 can also be replaced by a block of foamed dielectric material having a low dielectric constant. A similar set of annular members could also be used to suspend sleeve 12 within waveguide section 2 if necessary to maintain alignment of sleeve 12 with respect to the axis of section 2. In such case sleeve 12 would have to be slideably mounted in one set of annular members to permit axial motion with respect to waveguide sections 2 and 4. Sleeve 12 is also slideably mounted in a concentric center opening through bellows 10.

Waveguide sections 2 and 4 are designed for transmittal of some desired energy mode which will normally be the TE mode in long distance systems. One end of sleeve 12 causes the signal energy which is being transmitted through the waveguide in the desired mode to convert to another mode which propagates almost entirely within dielectric sleever 12 and is evanescent outside the sleeve. The particular mode, called a trapped mode, to which the energy is converted will depend upon the exact configuration of sleeve 12 and on the field configuration of the exciting mode. The signal energy propagates across gap 8 in the form of the trapped mode and is reconverted to the desired mode of propagation at the other end of sleeve 12. The attenuation of the trapped mode in sleeve 12 is substantially greater per unit length than the attenuation experienced by the desired mode of propagation in waveguide sections 2 and 4. However, the total amount of attenuation through sleeve 12 is small because of the relatively short length of the sleeve. The attenuation experienced is much preferable to the degradation experienced in other types of joints from the propagating mode being converted into numerous spurious modes by mechanical discontinuities in the joint.

As previously mentioned, the specific configuration of sleeve 12 will determine the mode of the energy propagated therethrough. The field configuration of the trapped modes will tend to be similar to that of the exciting field in waveguide sections 2 and 4. The ends of sleeve 12 advantageously have tapered cross-sectional areas 13 as indicated in FIG. 1 to insure a smooth and complete conversion from the energy mode in waveguide sections 2 and 4 to the trapped mode in sleeve 12. In most applications, the tapered ends of sleeve 12 will have a length at least as great as six to eight wavelengths ofthe lowest frequency of the mode being transmitted through sections 2 and 4. The taper on the ends of sleeve 12 might advantageously be on the order of 20 to l.

Sleeve 12 can be optimally placed in waveguide sections 2 and 4- to minimize the generation of spurious modes such as the TE mode which would degrade the signal. This is accomplished by placing the tip of the tapered ends at positions in waveguide sections 2 and 4 where the field of the undesired mode is at a minimum.

An alignment sleeve 16 is placed about the joined ends of waveguide sections 2 and 4 to complete expansion joint 6. Alignment sleeve 16 is rigidly mounted about the outer periphery of section 4 and is slideably mounted about the outer periphery of section 2 in order to permit relative axial motion of the two sections. The length of alignment sleeve 16 is determined in part by the required degree of axial alignment of sections 2 and 4 which must be maintained. The end of alignment sleeve 16 which is slideably mounted about section 2 advantageously might have a low friction surface thereon. Alignment sleeve 16 contains a recessed portion 18 which covers bellows l and permits movement thereof. The outer circumference of bellows is normally larger than the outer circumference of waveguide sections 2 and 4. Alignment sleeve 16 can be segmented to permit easier installation after sleeve 12 and bellows 10 have been installed.

Sleeve 12 may be replaced by a solid dielectric rod without any significant changes in the configuration shown in FIG. 1. The ends of a solid dielectric rod would be tapered to a point. The trapped mode which would be generated by such a solid dielectric rod will normally be different from the trapped mode generated by sleeve 12. A solid dielectric rod will also exhibit characteristics different from those of sleeve 12 in the generation of spurious energy modes.

Sleeve 12 can also be replaced by an artificial dielectric sleeve as illustrated in FIG. 2. The artificial sleeve is created by a succession of axially spaced dielectric membranes 20 having dipole patterns thereon. Membranes 20 are concentrically mounted in waveguide sections 2 and 4 and fastened around the interior periphery thereof. Across expansion gap 8 membranes 20 are concentrically fastened to bellows 22 by various mounting means 23 which will be apparent to those skilled in the art.

As shown in FIG. 3, membranes 25 near the ends of the artificial sleeve have a very thin layer of dipoles 24. This layer simulates the thin tapered ends of sleeve 12 shown in FIG. 1. The dipole pattern gets progressively heavier or thicker toward the middle of the artificial sleeve. The middle section of the artificial sleeve comprises a series of membranes 26 having equal thick layers of dipoles 28 as shown in FIG. 4 which simulate the middle sleeve section of constant thickness shown in FIG. 1.

Membranes 20 having dipole patterns may be formed by techniques known in the art. One convenient method of forming such membranes with dipole patterns is by printing small conductors on thin dielectric membranes by printed circuit techniques.

The artificial dielectric sleeve of FIG. 2 is electrically equivalent to sleeve 12 of FIG. 1. Also, most mechanical components utilized with the artificial dielectric sleeve are similar or identical to the respective components utilized with the sleeve of FIG. 1. Thus, there will be a choice of expansion joint designs for various situations.

While the invention has been described with respect to specific embodiments of expansion joints, it is to be understood that various modifications to the disclosed embodiments may be made by those skilled in the art without departing from the spirit and scope of the following claims.

What is claimed is:

1. An expansion joint for joining first and second axially aligned waveguide sections having an expansion gap between the adjacent ends thereof comprising, in combination:

a sleeve of dielectric material having ends with tapered longitudinal cross-sectional areas bridging said gap and ex tending a substantial distance into said ends of said waveguide sections so that said sleeve can propagate electromagnetic signal energy across said gap;

mounting means for concentrically mounting said sleeve of dielectric material within said waveguide sections so that said sleeve causes a transfer of electromagnetic signal energy between said sections and said sleeve at the respective ends of said sleeve;

flexible means for mechanically joining said ends of said sections so that said sections can move axially with respect to each other; and

alignment means for maintaining axial alignment of said sections across said expansion gap.

2. An expansion joint for joining first and second axially aligned waveguide sections having an expansion gap between the adjacent ends thereof comprising, in combination:

dielectric means bridging said gap and extending a substantial distance into said ends of said waveguide sections such that said dielectric means can propagate electromagnetic signal energy across said gap;

mounting means for concentrically mounting said dielectric means within said waveguide sections so that said dielectric means causes a transfer of electromagnetic signal energy between said sections and said dielectric means at the respective ends of said dielectric means;

a bellows joining said sections and isolating the interior of said sections from the surrounding environment; and

alignment means for maintaining axial alignment of said sections across said expansion gap.

3. An expansion joint for joining first and second axially aligned waveguide sections having an expansion gap between the adjacent ends thereof comprising, in combination:

a continuous length of dielectric material bridging said gap and extending a substantial distance into said ends of said waveguide sections such that said length can propagate electromagnetic signal energy across said gap;

a plurality of annular membranes of dielectric material surrounding said length, said annular membranes having the inner edge thereof fastened to the outer periphery of said length and the outer edge thereof fastened to the inner periphery of one of said sections so that said length is concentrically suspended within said section to thereby cause a transfer of signal energy between said sections, said length having tapered ends to reduce the loss of said signal energy during said transfer;

flexible means for mechanically joining said ends of said sections so that said sections can move axially with respect to each other; and

alignment means for maintaining axial alignment of said sections across said expansion gap.

4. An expansion joint for joining first and second axially aligned waveguide sections having an expansion gap between the adjacent ends thereof comprising, in combination:

a plurality of dielectric membranes having patterns of dipoles thereon bridging said gap and extending a substantial distance into said ends of said waveguide sections such that said membranes can propagate electromagnetic signal energy across said gap;

mounting means for concentrically mounting said membranes within said sections and across said gap at spaced points so as to cause a transfer of signal energy between said sections and said membranes, said dipole patterns simulating a sleeve of dielectric material with respect to said signal energy;

flexible means for mechanically joining said ends of said sections so that said sections can move axially with respect to each other; and

alignment means for maintaining axial alignment of said sections across said expansion gap.

5. Apparatus in accordance with claim 4 wherein said dipole patterns on said membranes across said gap are thicker than said dipole patterns on said membranes within said sections such that said dipole patterns simulate a tapered sleeve of dielectric material.

6. Apparatus in accordance with claim 4 wherein said membranes in said sections are attached to the peripheries of said sections, and said membranes across said gap are attached to 5 said flexible means. 

1. An expansion joint for joining first and second axially aligned waveguide sections having an expansion gap between the adjacent ends thereof comprising, in combination: a sleeve of dielectric material having ends with tapered longitudinal cross-sectional areas bridging said gap and extending a substantial distance into said ends of said waveguide sections so that said sleeve can propagate electromagnetic signal energy across said gap; mounting means for concentrically mounting said sleeve of dielectric material within said waveguide sections so that said sleeve causes a transfer of electromagnetic signal energy between said sections and said sleeve at the respective ends of said sleeve; flexible means for mechanically joining said ends of said sections so that said sections can move axially with respect to each other; and alignment means for maintaining axial alignment of said sections across said expansion gap.
 2. An expansion joint for joining first and second axially aligned waveguide sections having an expansion gap between the adjacent ends thereof comprising, in combination: dielectric means bridging said gap and extending a substantial distance into said ends of said waveguide sections such that said dielectric means can propagate electromagnetic signal energy across said gap; mounting means for concentrically mounting said dielectric means within said waveguide sections so that said dielectric means causes a transfer of electromagnetic signal energy between said sections and said dielectric means at the respective ends of said dielectric means; a bellows joining said sections and isolating the interior of said sections from the surrounding environment; and alignment means for maintaining axial alignment of said sections across said expansion gap.
 3. An expansion joint for joining first and second axially aligned waveguide sections having an expansion gap between the adjacent ends thereof comprising, in combination: a continuous length of dielectric material bridging said gap and extending a substantial distance into said ends of said waveguide sections such that said length can propagate electromagnetic signal energy across said gap; a plurality of annular membranes of dielectric material surrounding said length, said annular membranes having the inner edge thereof fastened to the outer periphery of said length and the outer edge thereof fastened to the inner periphery of one of said sections so that said length is concentrically suspended within said section to thereby cause a transfer of signal energy between said sections, said length having tapered ends to reduce the loss of said signal energy during said transfer; flexible means for mechanically joining said ends of said sections so that said sections can move axially with respect to each other; and alignment means for maintaining axial alignment of said sections across said expansion gap.
 4. An expansion joint for joining first and second axially aligned waveguide sections having an expansion gap bEtween the adjacent ends thereof comprising, in combination: a plurality of dielectric membranes having patterns of dipoles thereon bridging said gap and extending a substantial distance into said ends of said waveguide sections such that said membranes can propagate electromagnetic signal energy across said gap; mounting means for concentrically mounting said membranes within said sections and across said gap at spaced points so as to cause a transfer of signal energy between said sections and said membranes, said dipole patterns simulating a sleeve of dielectric material with respect to said signal energy; flexible means for mechanically joining said ends of said sections so that said sections can move axially with respect to each other; and alignment means for maintaining axial alignment of said sections across said expansion gap.
 5. Apparatus in accordance with claim 4 wherein said dipole patterns on said membranes across said gap are thicker than said dipole patterns on said membranes within said sections such that said dipole patterns simulate a tapered sleeve of dielectric material.
 6. Apparatus in accordance with claim 4 wherein said membranes in said sections are attached to the peripheries of said sections, and said membranes across said gap are attached to said flexible means. 